In a typical switching power supply that implements pulse width modulation (PWM), control loops have been implemented at an output of the supply and/or at a primary switch, for example, a field-effect transistor (FET). In general, inner-loop current feedback at the primary switch is desirable to provide stability and/or to detect overcurrent conditions. In a typical switching power supply, relatively expensive magnetic-field sensing hardware, such as a Hall-effect or magnetorestrictive sensor, or a very low-value power resistor, which is inserted in series with the switch, have been utilized to provide current feedback. When a resistor is utilized in series with the switch, the ramp voltage developed across the series resistor is then fed back to a control unit, which utilizes ramp slope information, along with an instantaneous output voltage, to control the modulation of a control signal applied to a control terminal of the switch.
Field-effect transistors (FETs) have been widely utilized as primary switches in switching power supplies. Various articles have proposed utilizing a saturation resistance, i.e., a drain-to-source resistance RDSon, of a FET to serve as a current sense resistor for the FET. However, the resistance of a typical conducting FET may vary over one-hundred percent over typical temperature operating ranges. As such, these temperature variable resistance changes may result in large errors in current feedback and, as a result, the current loop characteristics of a switching power supply implementing this technique are highly temperature sensitive.
Due to cost considerations, it would be desirable to develop a circuit for a switching power supply that measures a primary switch current economically and with enough accuracy to allow for adequate current loop control. What is needed is a technique for providing a temperature compensated feedback signal for a switching power supply.